1. Field of the Invention
The present invention is generally directed to an optical scanning apparatus adaptable for use in an electrostatic-latent-image forming apparatus, a carrier-transit-time measuring apparatus, or the like, and to an image forming apparatus, such as a digital copying machine, a laser printer, a laser facsimile, or a multifunction product providing two or more functions thereof, that includes the optical scanning apparatus.
2. Description of the Related Art
Conventionally, an optical scanning apparatus that forms a latent image by causing a light beam emitted from a light source, such as a laser diode (LD), to pass through a scanning optical system that includes a light deflection unit (e.g., an optical deflector, such as a polygon mirror) so as to form an image on a to-be-scanned surface of an image carrier (e.g., a photo-conductive photosensitive member) is known. Such an optical scanning apparatus is used as a latent-image forming unit of an electrophotographic image forming apparatus (e.g., a digital copying machine, a laser printer, a laser facsimile, a multifunction product that provides two or more functions thereof). Along with advent of high-speed, high-density image forming apparatuses, an optical scanning apparatus that includes a multi-beam scanning optical system that performs scanning with a plurality of light beams simultaneously to thereby write a plurality of lines in the sub-scanning direction simultaneously has been proposed.
A photosensitive member for use as a latent image carrier of an electrophotographic image forming apparatus can exhibit a reciprocity law failure phenomenon, in which even when the photosensitive member receives a same total exposure energy density, a state of latent image formed on the photosensitive member varies depending on a combination of light quantity and exposure duration. More specifically, the reciprocity law failure phenomenon occurs such that a change in electric potential on a photosensitive member that receives exposure of very short duration is smaller than that on a photosensitive member that receives exposure of relatively long duration even when total exposures thereof are equal to each other.
This is considered to be caused by an increase in the number of recombined carriers due to a large quantity of light, which causes a decrease in the number of carriers that reach a surface. With a multi-beam-scanning optical system, this results in uneven image density.
FIG. 19 illustrates an example where an image forming apparatus uses a 4-channel laser diode array (4ch-LDA), in which four laser diodes LD1 to LD4 are arranged, as a scanning optical system. Because a boundary area between the LD1 and the LD2 is exposed by both of them, the boundary area receives a large quantity of light during a short duration. In contrast, because a boundary area between the LD4 and the LD1 is exposed such that the LD4 is exposed first and thereafter the LD1 is exposed, there is produced a time lag that causes the boundary area to receive weak light for a long duration. In this case, a latent image formed by exposure with the time lag has deeper electric potential distribution and hence more likely to attract toner. Accordingly, image density at the boundary area between the LD4 and the LD1 becomes thicker than that at the other portions, which results in uneven image density.
The reciprocity law failure phenomenon as mentioned above particularly depends on, among characteristics of a photosensitive member, the thickness of a charge generation layer (CGL) of an organic photoconductor (OPC), carrier mobility, quantum efficiency, and the number of generated carriers, for example. Therefore, it is desirable to provide an image forming system that includes a photosensitive member and a scanning optical system that causes reciprocity law failure less likely to occur; however, a spatial resolution of as low as approximately several millimeters has been achieved with a conventional measurement method, which is insufficient for analysis of mechanism. Therefore, there has been no choice but to determine optimum exposure condition only based on an output image and light quantity has been adjusted so as to prevent uneven density based on the output image as a stopgap solution.
This method is also disadvantageous in that, because it requires adjustment of output power of each of light sources, when the number of the light sources increases, the number of combinations increases enormously, which not only makes it difficult to perform the adjustment but also makes it difficult to obtain an image stably.
An example conventional technique that aims at obtaining a high-quality image by preventing image quality degradation caused by reciprocity law failure even when scanning is performed with multiple beams is disclosed in Japanese Patent Application Laid-open No. 2004-77714. It is described that “employment of interlaced scanning allows, with any pair of neighboring scanning lines, a scanning number j of one of the pair to differ from the other one of the pair, thereby making scanning interval to be longer than a period of time of a single main-scanning stroke. As a result, degradation in image quality due to banding caused by reciprocity law failure can be reduced by a large extent and an image that is practically unlikely recognized as having image quality defect is obtained.”
The conventional technique disclosed in Japanese Patent Application Laid-open No. 2004-77714 employs interlaced scanning; thereby reducing degradation in image quality due to influence of banding caused by reciprocity law failure.
However, the conventional technique is disadvantageous in not taking characteristics of a photosensitive member, which is a latent image carrier, into consideration even though a main cause of reciprocity law failure is the characteristics of the photosensitive member.